The present invention relates to a method of decorative printing on a base material having a non-flat front surface through inkjet printing.
Vehicles include decorative members such as ornaments, emblems, and front grill garnishes. These decorative members employ a base material formed of transparent resin. The base material includes a front surface serving as a decorative surface and a back surface serving as a printing surface. A printing film is formed on the printing surface so as to be visible from the front surface of the decorative member through the base material.
To form a printing film, a screen printing method is typically employed. In a screen printing method, a squeegee is moved along a screen under pressure to squeeze ink out from the screen, thus applying the ink onto a printing surface. However, as more colors are used, more steps have to be carried out in the printing method to apply the ink onto the printing surface and cure the ink. Also, since the ink remains on the screen after printing, an excessive amount of ink is necessary, thus raising costs.
Further, screen printing is difficult to perform unless the printing surface is a flat surface or a curved surface having a uniform curvature. For example, if the printing surface includes a recessed portion, it is impossible to move the squeegee under pressure with the screen maintained close to the printing surface.
To solve this problem, it was thought to form a printing film by an inkjet printing method. In the inkjet printing method, ink droplets of different colors are ejected onto a printing surface. Then, ultraviolet rays, for example, are radiated onto the droplets to cure the droplets on the printing surface. This allows comparatively easy printing with fewer steps, using less ink, and regardless of the shape of the printing surface.
The inkjet printing method was originally designed to be performed on a flat printing surface, such as a sheet of paper. In this case, as illustrated in FIG. 8, an ink head 50 includes a plurality of ejecting portions 51 each including a nozzle 52. Each of the nozzles 52 moves while maintained close to a printing surface 53 of a decorative member 55. The distance X from the nozzles 52 to the printing surface 53 is constant throughout the area corresponding to the printing surface 53 and, specifically, 1 to 2 mm. In other words, droplets 54 ejected through each nozzle 52 travel over a short distance in a short time. As a result, the droplets 54 are unsusceptible to influence by the air or wind. This allows the droplet 54 to reach a target position on the printing surface 53 relatively accurately, as indicated by the lines in FIG. 8 composed of a long dash alternating with two short dashes lines. At this stage, the droplets 54 reach the position with limited variation.
Even if the printing surface 53 is a non-flat surface, that is, for example, a golf ball having slightly recessed portions in a surface is an object for printing on, as described in Japanese Laid Open Patent Publication No. 2006-75253, the distance from nozzles to the printing surface is short even for a portion corresponding to the bottom of each of the recessed portions. This ensures an advantage similar to the above-described advantage.
However, vehicle decorative members such as vehicle ornaments, emblems, or front grill garnishes each include both an area in which the distance X from the corresponding nozzle 52 to the printing surface 53 is short as indicated in FIG. 9 by the line composed of a long dash alternating with two short dashes line and an area in which the distance X from the nozzle 52 to the printing surface 53 is long as indicated by a solid line in the drawing. In this case, for the area corresponding to the shorter X distance, each droplet 54 travels over a short distance in a short time to reach the printing surface 53. This limits the influence on the droplets 54 by the resistance of air or wind, thus reducing the size of the variation range R of a droplet receiving position at which the droplets 54 are received by the printing surface 53.
However, as the distance X from each nozzle 52 to the printing surface 53 increases, the distance and the time for which each droplet 54 travels to reach the printing surface 53 increase. Accordingly, in correspondence with increase of the distance X, the influence on the droplet 54 by the resistance of air or wind increases to an extent that cannot be ignored. This may cause the droplets 54 to reach a position greatly offset from a target position on the printing surface 53, thus enlarging the variation range R of the droplet receiving position in excess of an acceptable range. As a result, the droplets 54 may reach a position outside a prescribed printing zone, thus causing an unclear boundary between the printing zone and a non-printing zone or between adjacent printing zones, that is, the definition of printing zones can blur.